Wang, B.; Ding, S.; Jiang, W.; Guo, X.; Han, R.; Zeng, L.; Wang, J.; Pedley, T. J.

Navigating fluid flow is a fundamental challenge for microbial life across diverse aquatic environments. While rheotaxis in swimming microorganisms has been extensively studied, it remains unresolved whether near-bed shear merely perturbs gliding motility or instead provides directional cues for active navigation on surfaces. Here we show that the benthic diatom Navicula cryptocephala utilises a purely mechanical strategy to achieve downstream rheotaxis and anisotropic spreading on submerged surfaces. Single-cell ellipsoidal tracking reveals a direction-dependent angular response that reorients gliding cells towards the downstream direction. Using interference reflection microscopy, we further reveal that shear induces rolling of obliquely gliding cells, laterally shifting the cell-substrate contact site. This shift renders raphe-based propulsion non-collinear with substrate friction, generating a downstream-restoring yaw torque. Crucially, our results rule out alternative explanations based on longitudinal shifts of the raphe contact site or direct hydrodynamic yaw torque. A minimal stochastic model confirms that this mechanical reorientation alone is sufficient to reproduce the observed drift and diffusion patterns, without invoking either orientation-dependent switching between motility states or orientation-dependent dwell times of those states. Our findings uncover a mechanism by which ambient shear is converted into directional guidance for active surface motility, providing new insights into microbial transport, retention, and resilience on submerged surfaces.